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All praise is due to Allah, and may His blessings and peace be upon His Messenger ﷺ, his family, and his Companions. To proceed:

If the adhān marked the actual beginning of Fajr and you knowingly swallowed another bite after that, your fast is invalid. Eating and drinking must stop when true dawn (al-fajr al-ṣādiq) begins, rather than when the adhān finishes. Allah establishes dawn as the end of the time permitted for eating and drinking before the fast (al-Baqarah 2:187).

Imām al-Nawawī explains that if dawn begins while someone has food in their mouth, they should spit it out and continue fasting. If they deliberately swallow it knowing that dawn has begun, their fast becomes invalid. (Al-Majmūʿ Sharḥ al-Muhadhdhab, Book of Fasting, discussion of food remaining in the mouth at dawn.)

However, an early adhān given before true dawn does not itself prohibit eating. The deciding factor is whether Fajr had actually begun. An adhān, when given reliably at the correct Fajr time, should be followed immediately.

If the bite was swallowed after true dawn:

• For a Ramaḍān fast: You must abstain from food, drink and other invalidators for the remainder of the day and make up that day. If you knowingly violated the fast without an excuse, you should also repent.

• For another obligatory fast, such as making up a missed Ramaḍān day: That day does not fulfil the obligation; you must fast another day.

• For a voluntary fast: That day does not count as a valid fast. According to the Shāfiʿī school, making it up is not obligatory. (Al-Māwardī, Al-Ḥāwī al-Kabīr, Book of Fasting, chapter on voluntary fasting.)

If you ate believing that dawn had not yet begun, but later established that it had, the Shāfiʿī position and that of the majority require making up an obligatory fast. This differs from deliberately eating with knowledge that Fajr has begun. (Al-Nawawī, Al-Majmūʿ, Book of Fasting, discussion of mistakenly eating before dawn.)

Allah knows best.